The use of fracturing fluids for well stimulation is well-known. Fracturing fluids are pumped down a well into a subterranean formation under hydraulic pressure until fracture of the formation is achieved. As a result, production of the well can be improved.
Proppants are typically added to the fracturing fluid before injection into the well. The proppants are useful in propping open the fractures produced during the process to maintain channels in the subterranean formation through which oil or other fluids can flow. It is beneficial if the fracturing fluid has a high viscosity, as this permits a large amount of proppant to be carried by the fracturing fluid.
One type of fracturing fluid comprises a polymer and a base liquid, e.g. an aqueous solution. The polymer is combined with the aqueous solution and hydrates, or gels, over a period of time, causing the fluid mixture to become more viscous. Proppants and other desired additives can be mixed with the hydrated, viscous fluid, which is then pumped into a well under hydraulic pressure.
One problem encountered with polymer-based fracturing fluids is the length of time required for hydration. Typically, ten minutes or more are required for the polymer to achieve full hydration, and hence, for the fluid to achieve full viscosity. As a result, large tanks are typically utilized to produce batches of hydrated fracturing fluids at the job site. Wasted fluids often result due to hydrating more polymer than is necessary to complete a particular fracture job.
Therefore, a need has arisen for methods to conduct continuous or "on-the-fly" fracturing fluid hydration. On-the-fly hydration is accomplished by continuously hydrating a sufficient amount of polymer for injecting into the well as required during the fracture job, as opposed to batch hydration, where the entire amount of fluid which is estimated to be necessary is hydrated in one batch prior to the job. On-the-fly fluid hydration provides a number of advantages including: ability to vary the amount of polymer added during the fracture process; losses due to unused fluid left in batch tanks are reduced; disposal costs for unused fluid are reduced; tank cleaning costs are reduced; on-site time is reduced; the need for bactericides and buffers is reduced; job sites are left cleaner since there are fewer spills of fluids and materials; and overall job efficiency and safety is improved by reducing the labor and efforts required by equipment operators prior to the actual pumping of the job.
A previous technique for on-the-fly fracture fluid hydration utilizes chemicals to eliminate the storage tank for holding the fluid during hydration. Anderson et al. U.S. Pat. No. 4,635,727, discloses a method for the formation of a hydrated gel from a guar gum by utilizing cross-linking agents which allow the gel to continue to hydrate in their presence. The disclosed cross-linking agents include zirconium lactate and aluminum chlorohydrate. The gel is pumped into the well before complete hydration is achieved and continues to hydrate in the well until full viscosity is reached. However, it is desirable that hydration be substantially complete before the fluid is pumped into the well in order to permit the addition of large amounts of proppants.
Adams, Jr., U.S. Pat. No. 4,716,932, discloses an apparatus for the gelation of a polymer fracturing fluid. Dry polymer is mixed with water in a storage tank. The mixture is then pumped through a long manifold to achieve hydration. The disclosed manifold is a 115-foot long section of 14-inch diameter pipe. However, a pipe of this size presents even greater cleaning problems and waste disposal problems than a batch tank. In other words, once the fracture job is complete, the fluid remaining in the pipe would require disposal, and the pipe itself would require cleaning.
Constien, et al., U.S. Pat. No. 4,828,034, discloses a method for continuously producing a hydrated fracture fluid from a slurried polymer during the fracturing of a subterranean formation. This method utilizes a high shear pump followed by a plug flow tank followed by another high shear pump and another plug flow tank. It is disclosed that substantially full hydration can be achieved in less than 5 minutes during this operation.
Although the method taught by Constien et al. provides a number of benefits over methods disclosed in the prior art, it also suffers from a number of disadvantages. For example, the high shear pumps employed in the method of Constien et al. are not efficiently located in the process. Also, a separate pump must be employed for each application of shear.
Therefore, it would be advantageous to have a continuous hydration method and apparatus that permitted hydration prior to introduction of the fluid into a well. It would also be advantageous if clean-up after the fracture job could be simplified and left-over fluid could be reduced to a minimum. Additionally, it would be advantageous if a method and apparatus could be provided for producing a continuously-hydrated fracturing fluid in an efficient and rapid manner.